Contact lenses are generally classified as hard contact lenses and water-containing soft contact lenses. Both types must be kept in their respective preserving solutions while they are not being worn, in order to preserve their delicate shape and performance. In particular, water-containing soft contact lenses should be stored in physiological saline to keep a constant water content thereby maintaining the characteristic softness that assures comfort in use.
Preserving solutions for contact lenses usually contain preservatives in concentrations of several parts per million to several percent, for the purpose of suppressing growth of bacteria and the like that enters through the opening of a container.
The preservatives must be harmless to the mucous membranes of the eyes and, at the same time, should not adversely affect the lenses. Benzalkonium chloride and chlorobutanol are effective antimicrobial agents currently and widely employed as such a preservative. They are used, of necessity, at concentrations safe to the eye mucosa. However, these preservatives are not suitable for certain water-containing soft contact lenses or hard contact lenses that have a high silicone content.
That is, in certain circumstances, the above-described preservatives penetrate into the inside of a lens and gradually concentrate in the inside. In the case of a water-containing soft contact lens, the inside concentration eventually reaches a level that might cause irritation to the eye mucosa. In the case of a high-silicone hard contact lens, the inside concentration is also problematical. Above all, the hydrophilic properties of the lens are reduced, which seriously impairs the comfort while the lens is being worn.
Water-containing soft contact lenses should be disinfected after use and before storage, to remove bacteria attached thereto while in use. Disinfection is usually effected by boiling. The microbicidal effect of hydrogen peroxide is also useful for disinfection.
However, in order to disinfect lenses by boiling, it is necessary for a user to procure a dedicated boiler and to secure a power source for operating the boiler, which is very inconvenient for a user especially when travelling.
This problem is avoided by using hydrogen peroxide instead of a boiler, but in this case a user must prepare two liquids, a preserving solution and a disinfectant, wherever he or she is (in places where such liquids are not available, a user must always bring the liquids with him or her). Further, disinfection with hydrogen peroxide should be followed by careful and certain neutralization of hydrogen peroxide remaining on the lenses.
Whichever disinfection technique is chosen, it has been an economical burden and an inconvenience for soft contact lens users to carry out storage and disinfection in separate systems.
In order to overcome these problems a solution for contact lenses with which disinfection and preservation (storage) can be effected simultaneously has recently been proposed, as an aqueous solution containing a polymer of a biguanide compound (see JP-A-61-85301 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")) and as an aqueous solution containing a polymer of poly(ammonium chloride) (see JP-W-A-83-501515 (the term "JP-W-A" as used herein means an "published Japanese national stage of international application")).
However, biguanide compounds tend to decompose during long-term storage. Unless used up in a short time after the preparation, the solution will reduce its crystalloid, which raises the concern that not only the disinfecting ability but also the storage stability of the solution is reduced.
Further, poly(ammonium chloride) does not have sufficient heat resistance. Since water-containing soft contact lenses must be sterile when shipped from a manufacturing factory, containers containing lenses are usually sterilized by autoclaving at a temperature of 120.degree. C. Poly(ammonium chloride) has a possibility of being decomposed by the high temperature. Eventually, two solutions, one for use in the sterilization and one for usual handling, should be prepared separately. Thus, a single solution is incapable of handling a contact lens from the shipment through the use.
An object of the present invention is to solve the above-described problems of conventional techniques and to provide a liquid for easy preservation and/or disinfection of a contact lens, which is applicable to any type of contact lenses and capable of preserving and disinfecting a contact lens, and which needs no special care after a disinfection.